AOI (abbreviated as Automatic Optical Inspection) is a technique used in PCB testing. It is the automated visual inspection of printed circuit boards. It is faster, more accurate and more reliable than manual visual inspection and allows for the production of quality products for shipping.

The need for AOI

PCBs are the root for electrical systems, one of the most basic requirement. Over time, PCBs have grown to become more complex and more compact. Even a simple board could consist of thousands of soldered joints. Overall, PCBs have become far too complicated to simply rely on manual visual inspection.

The demand for PCBs is also growing so for an effective line of business, it is important to keep up with the demand. But how to ensure that all your products are reliable and of high quality. Inspectors are not always reliable as they can easily miss errors in complicated boards, not to mention the time consumed. This is where AOI steps in the picture. AOI is not only fast but accurate and is considered an essential tool in production lines.

They are placed directly after the soldering process in the production line so they can detect faults early before they move along the production line. After all fixing errors further down the production line will cost more. A rapid response to errors means fixing the error before more boards are manufactured with the same faults.

The Basics of AOI:

Automated Optical Inspection is extremely reliable when looking for errors. They can detect a number of defects on the surface of PCBs such as nodules, scratches and stains and defects such as open circuits, solder shorts and thinning or missing solder. Also, incorrect components, components placed in the wrong position and missing components can also be detected easily.

This can be done by scanning the surface of the PCB by a high definition camera and light sources used for lighting up the PCB. The image captured is processed by the AOI system and is compared to the image provided to the AOI system. This is used to detect any defects or suspected faults in the manufactured board. There are three basic techniques used for this process.

Template Matching: Template matching is a technique used in digital image processing for detecting if the parts of an image match a provided template.

Pattern Matching: This technique is used to store information on correct and faulty assemblies in PCB manufacturing. The system also matches the image obtained from this information.

Statistical Pattern Matching:This technique is similar to template matching and pattern matching. The difference is that this method uses statistical data for inspection. It is designed for minor imperfections.

AOI systems are well built so as to detect all kind of defects and keep the number of errors found as minimum as possible after production.

AOI Image Capture & Analysis:

One of the main components of the AOI system is the image capturing system. This system captures an image of the PCB which is then used for inspection within the system to detect faults. There are many kinds of image capturing systems used in the AOI systems.

Image Capturing Systems can consist of a single camera or more and can even use 3D for better imaging. The cameras can be controlled through the software within the system. This way the camera is positioned to the desired position for image capturing. The type of camera used is also integral. It should be fast and accurate at the same time. The type of camera used can be:

Streaming Video: This type of camera used in AOI systems uses the streaming video application from which complete frames are taken. This frame is then used for a single image to be processed. This may not be entirely accurate but operates at high speed.

Still image camera system:This camera is used for capturing an image by placing it close to the PCB under a good lighting system. It is essential that the camera used can be controlled by the software within the system.

The AOI system can look for a number of features such as placement of components, size of components, size of components, label patterns etc. The soldered joints are also inspected to ensure the joints are free of errors. There are also many variations between good boards. For example, the size of components is not always the same for every batch. There can be differences in color type and thickness of ink used in silk screening.

Light Source in AOI:

Lighting is extremely important in the AOI system. A correct lighting source means better illumination of defects. A better lighting source means better detection of defects and therefore accuracy in production and speed. Usually, the lighting source is defined by the AOI system but can also be customized to requirements. Some types of lighting used include:

Fluorescent Lighting: This type of lighting is widely used in AOI systems and is very effective for viewing defects. The main problem is that the fluorescent lights degrade over time leading to constantly changing the quality of light.

LED Lighting:LED lighting is usually preferred over fluorescent lighting. This is because of it a more stable light source. Although it may degrade over time but can be adjusted by increasing current. The level of lighting can also be controlled easily.

Infra-red/ Ultra-violet lighting:Infra-red/ Ultraviolet lighting can also be used for enhancement of defects. This is basically for better detection of detects.

The positioning of lighting is also equally important in AOI systems. This is to ensure all areas are well-lit and defects can be detected easily. Positioning may also vary for different assemblies of PCB.

AOI System Programming:

The programming is also essential for a proper AOI system as it defines the whole operation of the system. All the details of the board manufacturing are stored in the system. This is where the defects are inspected. Methods for programming of AOI include:

Use of “Golden Board”: In this method, a properly manufactured board is provided to the system. The system, in turn, processes all the components, the joints and the solder profiles of the PCB. Usually, more than one board is used for providing all the required data.

Algorithm Based Programming: In this method, the system manufactures its own template for PCB inspection from the PCB data it is provided. Generally lesser amount of board is required for this method.

Both systems have their own pros and cons. The AOI can use either based on the requirements. It also depends on the amount of time required for production, the accuracy you are trying to achieve, and how to keep a balance between both.

Overall we can conclude that a proper AOI system focuses on accuracy and speed and proper detection of errors in the production line before there can be repeated errors further down the line. This is why AOI and systems like X-ray inspection are essential for a PCB manufacturing industry.

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